Refining of nitriles



Patented Nov. 19, 1946 ,UNi-TED STATES PATENT OFFICE John W. Teter and Walter J. Merwln,

Chicago,Iil.,

assignors to Sinclair Refining Company, New York, N. Y., a corporation of Maine No Drawing. Application October 6, 1943,

Serial No. 505,196

3 Claims. (Cl. 202-42) The crude, stabiother nitriles.

thus isolated nitriles are frequently contaminated by hydrocarbons which distill off within the same temperature range as the nitrile and which pass over with the respective nitriles from the fractionating operation in varying amounts, as an azeotrope.

The complete separation of such hydrocarbon impurities from these nitriles by direct fractional distillation has heretofore been extremely difiicult, if not impossible.

Our present invention provides a method whereby substantial complete removal of these hydrocarbon, impurities from these nitriles may be effected by direct fractional distillation.

We have discovered that the hydrocarbon contaminant of the nitrile fractions forms an azeotrope with acetonitrile in the presence of the otherrespective nitriles, which azeotrope'has a boiling point substantially below that of these other nitriles and may be separated therefrom by fractional distillation without substantial loss of the other nitriles.

The boiling point of pure propionitrile, for example, is approximately 97 C. and that of pure acetonitrile is 82 C. The azeotrope formed by acetronitrile and the hydrocarbon contaminants has been found to boil slightly below 82 C.. at atmospheric pressure.

Further, theacetontrile has been found not to form binary azeotropes with anymaterial quantity of either of the other nitriles previously noted herein, or ternary azeotropes with these nitriles and the hydrocarbon impurities, to any substan tial extent. A small amount of propionitrile may be carried over with the acetonitrlle but this amount is so small as to have little aifect on the efiiciency of the operationeven if not subse-,

quently recovered. Thus substantially complete removal of the added acetontrile and the hydrocarbon impurity from may be readily effected by distillation.

Utilizing this peculiarity of acetonitrile and the hydrocarbon contaminant to form a readily separable azeotrope in the presence of propionitrile and butyronitriles', the processin of our presentinventlon comprises adding acetonitrlle to the nitrile-hydrocarbon mixture and, fractionally distilling the mixture to separate the acetonitrlle-hydrocarbon azeotrope from the The optimum proportion of acetonitrlle to be added will depend primarily upon the proportion of the hydrocarbon contaminant present.

Most of the hydrocarbon readily separates fromthe acetonitrlle constituent of the azeotrope fraction as an upper layer upon cooling the fraction to room temperature and more may be removed by chilling. The lower acetoni'trile layer will contain most, if not all, of any propionitrile carried over with the azeotrope fraction and may be recycled with the decanted acetonitrlle to'the nitrile purification tower. The upper hydrocarbon layer will usually contain about 1.5% of dissolved acetonitrlle, at room temperature.

This acetonitrlle may be recovered by water washing, dried and returned to the nitrile purification tower.

The invention has also been found t be applicable to the direct purification of a stabilized mixed nitrile product resulting, for instance, from the catalytic amination of propylene. This product, as previously noted, usually contains various nitriles along with hydrocarbon impurities and sometimes contains small amounts of acetonitrile. In accordance with this aspect of my invention the hydrocarbon impurities may be removed directly from the nitrile mixture prior to isolation of the respective nitriles. Usuilly the amount of acetonitrile present, if any, is insuflicient to carry ofl completely the hydrocarbon impurities. Accordingly it is usually necessary to add to the mixture acetonitrlle from an extraneous source. However, we have found that the proportion of acetonitrile in the stabilized product can be regulated somewhat by the choice of the catalyst used and other operating conditions of the amination process. By this application of my invention the hydrocarbon impurities are distilled from the mixture with the acetonitrile and thereafter propionitrile and the butyronitriles substantially free from hydrocarbon impurities may be directly obtained.

Our present invention contemplates either' batch or continuous operation. In either type of the other nitriles noted mixture.

' operation, the acetonitrlle layer of the azeotrope the still, which will not be decomposed or distilled off at the distillation temperature of the an equal volume of acetonitrlle, no extraneous bottoms being added. The acetonitrlle-hydronitriles or of the azeotrope. and which will not interfere with separation of the acetonitrile-hydrocarbon azeotrope from the mixture. We have foundcapronitrile, for instance, to be satisfactory for this purpose. By-its use, danger of polymerizing and coking of the nitrile being distilled is substantially avoided and a more complete recovery of the nitrile is made possible. However, it will be understood that the use of such extraneous bottoms material is not essential to our purification process.

The invention will be illustrated by the following specific examples:

' Example I A synthetic mixture was prepared from 20 cc. of pure propionitrile, having a refractive index of 1.3662 at 20 C., and 2.8 cc. of hydrocarbons separated from hydrocarbon-contaminated propionitrile by chilling it. To this mixture there was added 50 cc. of acetonitrile and the entire mixture was subjected to fractionaldistillation in a 9.5 millimeter Stedman column. The acetonitrile-hydrocarbon fraction distilled off-at a substantially constant temperature beginning at 78 C. and finally rising to about 81C., indicatingexhaustion of the hydrocarbon contaminant. The propionitrile fraction distilled off at about 96 C. and was foundlto be substantially free from hydrocarbon impurities as indicated by a nitrogen content of 24.6% and a refractive index of 1.366 at 20 C., which is substantially that of the propionitrile used in the preparation of the synthetic mixture.

Example II 4160 cc. of a normal butyronitrile'product, contaminated with hydrocarbons and having a refractive index at 20 C. of 1.3850 and a nitrogen content of 19.3%, was subjected to fractional distillation in a two inch Stedman column with an equal volume of acetonitrile. In this distillation,

1930 cc. of capronitrile also was added toact as an extraneous bottoms. The acetonitrlle-hydrocarbon fraction distilled off at a temperature of 80 to 81 C., the temperature finally rising gradually to 82 C. for a brief period indicating exhaustion of the hydrocarbon impurity from the Thereafter the temperature rose sharply to 114 C. and then gradually to 118 C. In this last mentioned temperature range over of the charge, excluding the extraneous bottoms, was obtained and this fraction was found to contain approximately 20% nitrogen and to have a refractive index of 20 C. of 113840, indicating it to be substantially pure normal butyronitrile.

Example III 1390 cc. of an iso-butyronitrileproduct, contaminated with hydrocarbon and having a refractive index at 20 C. of 1.3737 and containing I gravity was found to be 0.7712.

'carbon fraction came ofi at a temperature of 4 80.5-81 C., the temperature remaining substantially constant until about of the charge was distilled off and then rapidly rising to 101 C. and finally gradually rising to 102 C. Therefractive index of the acetonitrlle-hydrocarbon fraction at 20 C. was 1.3462 but toward-the end of the cut dropped to 1.3441, approximating that of pure acetonitrile and indicating that the hydrocarbon impurities of the mixture had become exhausted and that more than the required amount of acetonitrlle had been added. The ni-- trogen content of the iso-butyronitrile fraction was-19.3% and its refractive index was 1.3731, substantially that of pure iso-butyronitrile, indi-. cating complete removalof the hydrocarbon impurities therefrom. About 820 cc. of the pure isobutyronitrile was thus obtained, Its specific Example IV To 100 cc. of a crude stabilized mixed nitrile product, resulted from the direct aminationof propylene there, there was added 50 cc. of acetonitrile, The mixture was then subjected to fractional distillation in a 9.5 millimeter Stedman column.

Approximately 44 volume percent of the mix-- ture distilled off at a temperature below 82 0.,

including 7.5% of material distilling oif below 75 C. Between the point where 7.5 to that where 29 volume percent of the charge was distilled off, the distillate separated into two layers upon cooling to room temperature. The refractive index of the upper layer at 20 C. rose from 1.412 to 1.420. Thereafter the distillate no longer separated upon cooling to room temperature and the refractive index of the distillate fraction dropped sharply to 1.351 and finally to 1.3450 at a temperature of 82 C., indicating that the hydrocarbon impurities, originally present in, the mixture,

had been substantially exhausted and relativelypure acetonitrlle was being distilled off.

After about 44 volume .per cent had been distilled off the temperature rose sharply to 94 C. and at 94 to 96 C. a fraction was obtained having a refractive index of l.3655'and a, nitrogen content of 24.1%, indicating that it was relatively pure propionitrile. Finally at 116.5 C. a fraction was obtained having a refractive index at 20 C. of 1.3840 and containing 19.4% nitro- 5 gen, indicating that it was substantially pure normal butyronitrile. Thus, by our improved process, the respective nitriles, uncontaminated by hydrocarbons may be obtained directly from a crude mixture of the nitriles containing hydrocarbons.

The hydrocarbon contaminant normally pres ent in these nitrile products has been found by analysis to consist of about 12.6% paramns, 64%

oleflns, 20.6 naphthenes, and 2.8% aromatics. 5 The identity and composition of the hydrocarbon contaminants may vary somewhat. However, we have consistently found them to be substantially completely removable from propionitrile or the butyronitriles or mixtures thereof by out im-" proved distillation process.

The invention is specifically illustrated in the an operation wherein the distillation proceeds 5 only to the point where the acetonitrile-hydrocarbon fraction has been distilled oil.

We claim:

1. In a process for removing from nitriles of the class consisting of propionitrile, normal butyronitrile, and isosbutyronitrile, hydrocarbon impurities having boiling points. such that in mixtures with said nitriles they normally distil in approximately the same temperature, range as said nitriles, the improvement which comprises subjecting the hydrocarbon-contaminated nitrile product to fractional distillation in admixture with acetonitrile and distilling oil! the hydrocarbon-contaminant and acetonitrile as an azeotrope. 0

2. In a process for removing from nitriles of the class consisting of propionitrile, normal butyronitrile and iso-butyronitrile, hydrocarbon impurities having boiling points such that in mixtures with said nitriles they normally distil in '20 approximately the same temperature range as said nitriles, the improvement which comprises adding acetonitrile to the hydrocarbon-contaminated nitrile and distilling oi! the hydrocarboncontaminant and acetonitrile as an azeotrope.

3. In a process for removing from nitriles 0'! the class consisting of propionitrile, normal butyronitrile and iso-butyronitrile, hydrocarbon impurities having boiling points such that in mixtures withis'ai'd nitriles they normally distil in approximately the same temperature range as said nitriles, the improvement which. comprises subjecting the hydrocarbon-contaminated nitrile product to fractional distillation in admixture with acetonitrile, distilling oil a fraction compris ing acetonitrile and the hydrocarbon-contaminant, cooling said traction; whereby the distillate is separated into an upper hydrocarbon layer and a lower acetonitrile layer and returning the lower layer to the zone of fractional distillation.

JOHN W. TETER.

WALTER J. MERWIN. 

